Original paper

Thermodynamic properties of magnesiochloritoid

Grevel, Klaus-Dieter; Kahl, Wolf-Achim; Majzlan, Juraj; Navrotsky, Alexandra; Lathe, Christian; Fockenberg, Thomas


Calorimetric and P-V-T data of synthetic magnesiochloritoid (MgAl2SiO5(OH)2) have been obtained. The P-V-T behaviour of monoclinic and triclinic magnesiochloritoid has been determined in situ up to 8.5 GPa and 800°C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with vaseline to ensure hydrostatic pressure transmitting conditions; NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus of the triclinic polytype was determined as 127.9 ± 2.1 GPa, (K′ = 4), VT,0 = 456.58 Å3 exp [∫(0.304 ± 0.022) × 10-4 dT], (∂KT/∂T)P = -0.017 ± 0.009 GPa K-1. The resulting fit parameters for the monoclinic polytype are very similar.The enthalpy of drop-solution was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr-University Bochum, Germany) using lead borate (2 PbO·B2O3) at 700°C as solvent. The resulting values were used to calculate the enthalpy of formation from the elements; -3538.9 ± 4.9 kJ mol-1 (Davis) and -3543.4 ± 6.2 kJ mol-1 (Bochum) were obtained.Heat capacity measurements of MgAl2SiO5(OH)2 were obtained by differential scanning calorimetry (DSC) in the temperature range from -10°C to 295°C. Two runs confirmed heat capacity data of Koch-Müller et al. (2002), represented by the Berman & Brown (1985) type four-term equation: CP = (391.75 -2585.00 × T-0.5 -8240000.0 × T-2 + 967000000.0 × T-3) J K-1mol-1.Consistency of the thermodynamic data obtained for magnesiochloritoid with phase equilibrium data reported in the literature was checked by mathematical programming analysis. The best agreement was obtained with ΔfH0298 (magnesiochloritoid) = -3551.7 kJ mol-1, and S0298 (magnesiochloritoid) = 142.2 J K-1mol-1.